How to convert Nm3/h of Air to Nm3/h of another gas?

Hi, I have calculated the thermal venting of a tank using API 2000 and have found the venting rate in nm3/h of AIR. I now need to convert it to blanketing gas. can i assume that the flowrates are equal or do i apply F2=F2x (SG2/SG1)^0.5 ? does this apply only to nozzles or can it be used on any fluid in any situation? help slightly confused.